This invention relates in general to transmissions for transmitting rotary motion and, more particularly, to an output split and compound split infinitely variable transmission.
The primary function of a transmission in an automotive vehicle is to convert the speed and torque of an engine or prime mover into the speed and torque required by the specific requirements of the vehicle. In general it is desirable for this power transmission to occur with minimal power loss; that is, at high transmission efficiency. To optimize the overall power train efficiency, specifically the engine-transmission combination, it is also desirable to minimize or to eliminate drive-line torque interruptions as well as engine speed excursions while delivering the desired speed and torque.
Transmissions can be broadly categorized as “step-less” or “stepwise”, according to the way that the final desired speed ratio is achieved. A stepwise transmission has a fixed number of speed ratios that are sequentially selected to achieve the final desired ratio. Therefore, it is not always possible to use the maximum available engine power, because engine speed and load vary as the transmission moves through its limited number of fixed ratios. Moving from one ratio to another is sometimes termed “shifting” in transmission design descriptions, and is often associated with unfavorable torque interruptions. Such interruptions offset overall drive-line efficiency and detract from driving comfort. Moreover, for some applications, such as agricultural and construction vehicles, an output torque interruption is extremely undesirable, more so than a momentary loss of efficiency. Such inherent shortcomings, associated with shifting, are minimized to some extent in more recent designs that incorporate pre-shifting and clutching to reduce torque interruptions. To reduce engine speed and torque variations, modem transmissions use a greater number of selectable ratios.
In principle, a stepwise transmission cannot achieve the overall vehicle efficiency of a step-less transmission which offers an infinite number of speed ratios. With a step-less transmission, there is the option to operate the engine at its optimum efficiency or lowest emission point at all times while the vehicle is moving to its desired speed at a desired power level.
Step-less transmissions can be further classified into continuously variable transmissions (CVTs) and infinitely variable transmissions (IVTs). A continuously variable transmissions, usually mechanical, provides continuously variable speed ratios over the designed speed range of the vehicle. A launch-clutch and engine-disconnect device is often required in this type of transmission for vehicle start-up, as well as a separate gear for reverse operation. In addition, most of the CVT designs transmit torque through contacting friction surfaces and are not suitable for high-torque and high-power applications.
The infinitely variable transmission, by definition, is capable of providing infinitely selectable (output-to-input) speed ratios from reverse, thru zero, to a wide range of forward speeds. No launching device is required for an IVT. In theory, the engine can be directly connected to the transmission at all times, because a zero output-to-input speed ratio or an infinite input-to-output speed ratio exists.
Historically, there were two types of IVTs that have had success in the marketplace; the hydrostatic transmission and the hydro-mechanical transmission. Hydrostatic transmissions provide a hydrostatic power path in which the input power is transformed into hydraulic power by a pump. The hydraulic power is then converted back to mechanical power by a hydraulic motor. Speed regulation of the hydraulic motor provides control of the desired vehicle speed. However, hydrostatic transmissions are very inefficient, particularly under partial load and slow speeds, and are often noisy. They are primarily used for applications where flexibility and speed-control are more important than efficiency.
The hydro-mechanical transmission concept significantly improved efficiency by providing a mechanical power path in addition to the hydrostatic power path, the mechanical path being the most efficient. This concept is known as power-split. Thus, the hydro-mechanical transmission using the power-split concept offers much improved efficiency compared to a hydrostatic transmission.
The power-split concept has been recently extended to include electric power paths rather than hydraulic paths to create electromechanical power-split systems. In an electromechanical transmission, the variators are electrical machines, namely motor-generators, rather than hydraulic machines. The advantages of using motor-generators rather than pumps and hydro-motors include system design flexibility, controllability, and improved performance in terms of a wider range of speed, better efficiency, and reduced operating noise.
When the speed ration goes beyond a certain range, the power in the electric path can actually exceed several times the power that is transmitted through the transmission. This phenomenon is known as internal power circulation. It wastes power, generating unwanted heat within the transmission.